QmlJS checks: Fix collection of use-before-declaration warnings.

Previously they would not be collected if the var had a
duplicate-declaration warning.

Change-Id: I82244e911fcdfc3d11cc391288da263ee288a3ca
Reviewed-on: http://codereview.qt-project.org/4330


Reviewed-by: Fawzi Mohamed <fawzi.mohamed@nokia.com>